This invention relates to improvements in calendering belts for use in a calendering apparatus for papermaking.
In the manufacture of paper, calendering is carried out by a calendering apparatus, which is a rolling machine used in a latter stage in the papermaking process, after raw material is processed in a papermaking machine to produce a web, and the web is compressed and dried. The purpose of the calendering process is to make the surface of paper smooth, to impart a gloss to the paper, and to make the thickness and density of the paper uniform. Calendering is indispensable for improving the quality of paper. When the surface of paper is rugged rather than smooth, or its thickness varies from one location to another, the appearance of the paper is unsatisfactory, and uneven printing can occur on the surface of the paper in the printing process. Moreover, if the density of paper is not uniform, even if the surface is smooth, printing performance will be impaired, because the rate of ink absorption will vary from one location to another.
A calendering apparatus makes the surface of paper smooth by the application of pressure to the paper by a pressurizing means having a smooth surface. Known calendering apparatuses include a xe2x80x9cmachine calenderxe2x80x9d that uses a pair of rollers made of steel, and a xe2x80x9csuper calenderxe2x80x9d having multi-stage pressurizing means composed by rollers made of steel and rollers having elastic covers.
In the case of the machine calender, the pressure cannot be relieved in the pressurizing part of the machine composed of the rollers, because both rollers are made of steel. Moreover, because the rollers come into contact with each other along a line, a large pressure will inevitably be applied along that line to the paper. A problem encountered with the machine calender is that, if a rugged paper is processed with such an apparatus, a relatively low pressure will be applied to the concave portions while a relatively high pressure will be applied to the convex portions. After the rugged paper passes through the machine calender, the areas corresponding to the concave portions will have a lower density than the areas corresponding to the convex portions. As a result, the density of the entire paper cannot be made uniform.
On the other hand, in case of a super calender, even in if the paper has a rugged surface, large pressure will not be applied locally because the use of the elastic cover results in an enlargement of the contact area of the two rollers. They contact each other over a relatively wide area rather than on a narrow line. As a result, in the case of a super calender, pressure is applied to the paper uniformly, and excessive pressure may be relieved due to the deformation of the elastic covers.
However, in case of the super calender, heat is likely to accumulate between the elastic cover and the roller while in use, and consequently the elastic cover is likely to deteriorate, and may eventually separate from the roller.
The above-mentioned problem, has been addressed by a calender apparatus utilizing an endless calendering belt having elasticity, as shown in FIGS. 5(a) and 5(b).
FIG. 5(a) shows an apparatus in which the pressurizing part is composed of a calender roller CR and a pressing roller PR, and FIG. 5(b) is an apparatus in which the pressurizing part is composed of a calender roller CR and a shoe S. In both cases, an endless calendering belt 100 and the paper W, which is subjected to the calendering process, are sandwiched in the pressurizing part.
The surface of the calender roller CR which comes into contact with the paper W is smooth and is heated to about 100xc2x0 C. to 200xc2x0 C. by a heating apparatus (not shown).
When a paper W, having a rugged surface resulting from the papermaking process, passes the pressurizing part of the calendering apparatus of FIG. 5(a) or FIG. 5(b), the surface of the paper W that contacts the calender roller CR is pressed by the calender roller and made smooth by heat and pressure. However, the opposite side, i.e., the back, of the paper W is not made smooth, because the calendering belt 100, with which it comes into contact, deforms elastically, following the ruggedness of the paper W.
Therefore, these calendering apparatuses can make one surface of the rugged paper W adequately smooth, and the density of the paper W will not exhibit marked local variations.
The calendering apparatuses of FIGS. 5(a) and 5(b) also have the advantage of excellent durability, because the calendering belt 100 is relatively long, and the heat generated in the pressurizing part is radiated efficiently.
In the calendering belt 100 used in a calendering apparatus shown in FIGS. 5(a) and 5(b), it is necessary that the web side surface WP, which comes into contact with the paper web W, be flexible, and that the pressing side surface MP, which comes into contact with the pressing roller PR or the shoe S, have good durability and wear resistance.
Based on the foregoing considerations, calendering belts have been designed in which the layer on the web side is made of a comparatively flexible high molecular weight material, and the layer on the pressing side is made of a high molecular weight elastic material which is comparatively hard. Such a belt is disclosed in Unexamined PCT Publication No. 501852/1998. Another calendering belt, in which the high molecular weight elastic material of the web side layer contains bubbles so as to increase its flexibility, is disclosed in Unexamined Japanese Patent Publication No. 88193/1985.
The calendering belt 100, which consists chiefly of high-molecular elastic material, lacks adequate heat resistance, and the portion which comes in contact with the calender roller CR readily deteriorates as a result of exposure to heat. The heat of the calender roller CR is intercepted by the paper W, and is not transmitted to the calendering belt 100 which is on the back side of the paper W. However, because the width of the calendering belt 100 is usually greater than the width of the paper W, both margins of the calendering belt 100 will come into direct contact with the calender roller CR at a high temperature in the pressurizing part of the apparatus. As a result, at both margins of the calendering belt 100, cracks may be generated, and wear becomes more severe due to deterioration and distortion caused by heat.
It is possible to use heat-resistant resins to improve the heat resisting property of the calendering belt 100. But in that case, premature wear of the pressing side WP becomes a problem, because heat resistant resins are generally inferior in durability.
Although fluorocarbon resins such as PTFE (polytetrafluoroethylene) are known as high molecular weight elastic materials having excellent heat resistance and durability, they are expensive, and therefore it is not realistic to manufacture a relatively long calendering belt 100 from such materials.
Because of the foregoing considerations, heretofore, it has not been possible to achieve durability, wear resistance and heat resistance simultaneously at a relatively low-cost in conventional calendering belt.
The improved calendering belt in accordance with the invention is an endless belt comprising a main central portion, and right and left portions located along the right and left sides of the main central portion, said right and left portions comprising high molecular weight elastic material with heat resistance greater than the heat resistance of the main central portion, and said main central portion comprising high molecular weight elastic material having durability higher than the durability of the right and left portions. The improved endless calendering belt also preferably has reinforced edge portions located along the outer edges of the right and left portions, the reinforced edge portions comprising high molecular weight elastic material having durability higher than the durability of said right and left portions.
The right and left portions do not readily deteriorate as a result of heat, even if they come into direct contact with the hot calender roller, because the heat resistance of the right and left portions is higher than that of the main portion. The main portion is not worn out easily even if it is repeatedly compressed in the pressing part of the calendering apparatus, because the durability of the main portion is higher than that of the right and left portions.